Shutdown, bilayer battery separators are known. For example, see U.S. Pat. Nos. 4,650,730; 4,731,304; 5,240,655; 5,281,491; and Japanese Kokai No. 6-20671, each of the foregoing is incorporated herein by reference.
In batteries, the anode and cathode are separated from one another by a separator. Today, "lithium batteries" are very popular because they are able to generate high energy outputs. The lithium battery market can be divided into two groups, the "primary" lithium battery and the "secondary" lithium battery. The primary lithium battery is a disposable battery, while the secondary lithium battery, is a rechargeable battery. A problem associated with the secondary lithium battery is its potential for short circuiting. Short circuiting arises when the separator ruptures and allows the anode and the cathode to come into direct electrical communication with one another. This short circuit may manifest itself with a rapid evolution of heat. This rapid evolution of heat can cause the battery to explode. Accordingly, the shutdown battery separator was developed.
The shutdown battery separator generally comprises two polymerically dissimilar and juxtaposed microporous membranes. One microporous membrane is chosen for its relatively low melting point and the other for its relative strength. For example, the low melting point membrane may be a polyethylene material and the strength membrane may be a polypropylene material. The polyethylene microporous membrane has a melting point of about 130.degree. C. This is sufficiently low that, in the event of a short circuit in a lithium battery, the heat generated will melt the polyethylene so that it shuts down, or fills in the pores of the separator, and thereby stops or inhibits the likelihood of a short circuit. The polypropylene membrane, which has a substantially higher melting point, approximately 160.degree. C., provides strength to the separator so that it maintains the separator's integrity in the event of a short circuit.
In U.S. Pat. No. 4,650,730 and 4,731,304, shutdown, bilayer battery separators of the foregoing type are disclosed. In the examples, separator thicknesses of 3-4 mils are disclosed. The methods of making these battery separators are disclosed as: 1) two discrete films containing extractable fillers are made, then bonded together, and then the fillers are extracted; 2) a bilayer film, containing extractable fillers, is coextruded and the fillers are subsequently extracted; 3) one film made by an extrusion, annealing and stretching process, and a second film made with extractable fillers are bonded together, and then the fillers are subsequently extracted; 4) a first film is coated with a second material. In U.S. Pat. No. 5,240,655, a shutdown, bilayer battery separator, of the above-mentioned type, is made by coextruding the discrete layers, and then stretching and annealing the extruded bilayer film. In U.S. Pat. No. 5,281,491, the shutdown, bilayer battery separator, of the above-mentioned type, is made by coextruding the first and second films that contain an extractable material, and the extractable material is subsequently removed.
In Japanese Kokai 6-20671, a shutdown, bilayer battery separator, of the above-mentioned type, has a thickness of about 1 to 2 mils, a penetration energy (from FIG. 1) of about 1660 g-mm (this value is equivalent to about 1800 g-mm when converted into units comparable with the test method disclosed herein), and a peel strength of 0.1 to 1 gram/centimeter. The separator is made by a process in which two discrete films, made by the process set forth in U.S. Pat. Nos. 3,679,538 or 3,801,404, are bonded together by calendaring at a temperature of 134.degree. C.
A problem with the foregoing battery separators is that, while they provide the necessary shutdown characteristics, they are deficient in one or more of the following qualities: thinness or penetration energy for puncture strength) or peel strength. In the manufacture of batteries, it is important to have extremely thin separators, so that: the energy density of the battery maybe increased; and the size of the battery, as well as, the electrical resistance across the separator, may be reduced. Good penetration energy is important in battery manufacture, particularly in the manufacture of "jelly roll" type batteries because the surfaces of the anode and the cathode can be sufficiently rough that they can puncture these extremely thin separators during manufacture. Good peel strength is important in battery manufacture because it prevents delamination of the separator. Accordingly, there is a need to produce an extremely thin, shutdown battery separator that has a sufficient penetration energy and peel strength to withstand the rigors of battery manufacture.